Cholesteric liquid crystals exhibit a helically twisted molecular orientation resulting in special optical properties. When a cholesteric LC is irradiated with unpolarized light, interaction of the helix structure with incident light of a selected wavelength will result in reflection of 50% of its intensity as circularly polarized light of a given handedness (left-handed or right-handed according to the handedness of the helix) while the other 50% are transmitted as circularly polarized light of the opposite handedness. The wavelength .lambda. of the reflection maximum depends on the pitch p of the helix and the average refractive index n of the cholesteric LC material according to the following formula: EQU .lambda.=n.p
Since the color effect of cholesteric optical materials is based on selective light reflection and not on absorption like in conventional dyes or pigments, extraordinary color-properties can be obtained like for example higher color saturation, wider color range and iridescent appearance. These materials exhibit a unique reflection pattern, because the reflected wavelength will change if the incident light propagates through the cholesteric LC at an angle to the direction of the helix axis.
However, to achieve good color properties when applied in inks or paints, a uniform alignment of the cholesteric LC with the orientation of the helix axis parallel to the viewing direction is required. Furthermore, low molar mass cholesteric LC's are best used in most applications in the liquid state if they are confined to small capsules or droplets. In this case the temperature dependence of the reflected wavelength is another problem. On the other hand polymeric cholesteric LC's which are used in the solid state have to be aligned above their glass transition or melting temperature respectively which requires high temperatures. Both embodiments are therefore inconvenient for production and limited in their applications.
By making flakes or platelets of prealigned cholesteric polymer material several problems of the prior art can be circumvented. To produce such flakes a cholesteric polymer material is coated onto a substrate and aligned to achieve uniform orientation of the helical axis normal to the surface. The film is then cured and ground to yield small flat flakes which can be dispersed e.g. in a transparent binder for the use as inks or paints. These inks can be used at room temperature without the need of further alignment.